Mobile and wireless telephony has grown enormously the last years, and it is expected to continue to grow all over the world. In addition, more and more services are added to the mobile phones, such as access to Internet. In a mobile communication system the mobile phones are terminals, and the network is formed by wireless connection of each terminal to the nearest fixed base station, where the base stations are connected together with other base stations in a fixed network. The next years we will also see a rapid growth of short-range microwave links, such as Bluetooth systems. In a Bluetooth system two terminals, which are located up to 100 m apart, communicate directly with each other.
Bluetooth and similar wireless short-range communication links are intended to replace all existing signal cables, e.g. the mouse, keyboard and network cables of a PC, and the headset cable of a mobile phone. Bluetooth will also open numerous new possibilities to locate sensors, and transfer signals from sensors, as well as from and to control units in machines and factories.
Many of the above-mentioned terminals are located in a so-called multi path environment. This means that the electromagnetic waves (which are modulated with the signal) will take many simultaneous paths between the transmitting terminal (or base station) and the receiving terminal. These paths are caused by reflections from objects like buildings, walls, trees, cars and furniture, as well as from human beings and animals.
The antennas on the terminals are therefore preferably designed for operation in multipath environment. This means that the shape of the radiation pattern plays a minor roll. The most important performance parameter is for the transmit case the radiation efficiency. The higher radiation efficiency the better, i.e., the more power is radiated out of the terminal antenna the better. The radiation efficiency has three main contributions: the transmission efficiency due to reflections (i.e. mismatch) at the antenna port, the efficiency reduction due to ohmic losses in the antenna itself, and the efficiency reduction due to ohmic losses in the near-in external environment of the antenna. This near-in environment may change depending on how the antenna or terminal is operated. An example is the loss in the human head and hand during operation of a mobile phone. The relative distribution between the three contributions to the radiation efficiency, as well as the total radiation efficiency itself, will change with the environment.
The radiation efficiency was explained above for a transmitting terminal antenna, but the same terms are used and valid for receive antennas, due to reciprocity.
Traditionally the radiation performance of antennas is measured outdoors or in anechoic chambers. The antenna under test is mounted on a turntable at one end of the measurement chamber or range, and there is a transmit antenna tower at the opposite end. The radiation pattern is obtained as the transmission between the two antennas as a function of the rotation angle of the turntable. In order to obtain the radiation efficiency, we need to measure the radiation pattern in all directions in space and integrate the received power density to find the total radiated power. This will then give the radiation efficiency when compared with the corresponding power integral of a reference antenna. This traditional measurement set-up requires expensive equipment, much work, and the final result is obtained after a long measurement procedure. In addition, the measured radiation patterns are not of much interest for terminal antennas.
As already explained the radiation efficiency depends on the near-in environment in which the antenna is located. Therefore, the terminal antenna needs to be tested out in such environments, e.g., a mobile phone antenna needs to be tested out at different positions and orientations relative to the human hand and head. Different terminal antennas will behave differently in different environments, and a good antenna will keep high radiation efficiency in different environments. Therefore, a lot of measurements of terminal antennas in different environments need to be done. This is very time-consuming and expensive with traditional radiation pattern measurements.
Similar problems as related above are encountered when measuring the total radiated power from the whole mobile or wireless terminal, such as from a mobile phone. Further, it is a problem with known measuring equipment to make a quick, easy and/or reliable estimation of the absorption of electromagnetic radiation in the human body. This is important, due to possible health effects, but also because absorption reduces the total radiated power outside the human body, which is used to communicate. In a mobile phone, we refer to this outer radiated power as the telephone communication power (TCP or simply CP). This is the total power radiated by the phone minus the power lost in the human body. The larger this communication power (CP) is, the better will the phone work when it is located in an environment where the signal from the base station is low. Existing equipment that is used to measure radiation into the human body is very expensive and laborious to use.
In respect of receive performance, this is normally either characterized by a Bit-Error-Rate (BER) or a Frame-Error-Rate (FER), depending on which system terminals are designed for, where the latter frame consist of several bits that are coded in a special way to reduce errors. The BER or FER will depend on the signal level present at the receiver. Therefore, the receiver sensitivity is defined as the level which provides a certain BER or FER, often chosen to be 0.5%. It is known how to measure the receiver sensitivity when a signal is connected directly to the port of the receiver of the terminal. This often referred to as conductive measurements because the transmit signal is connected directly to the receiver without including any antenna or environment. Then, however, the performance of the antenna is not included in the measurements. Therefore, it has been described how to measure the receiver sensitivity in an anechoic chamber. This is done by using a base station emulator connected to the transmit antenna in the chamber, and locating the terminal on the turntable. The receiver sensitivity for a certain BER or FER is then determined by analyzing the received signal at the phone, at each of all the directions of incidence on the terminal. The latter directions are obtained by moving the turntable in the anechoic chamber. These receiver sensitivities will vary much with direction, because the receive radiation pattern of the terminal is different for the different directions. Therefore, these values are averaged over all directions (which should be uniformly distributed over the complete unit sphere around the terminal). The averaged results are called a Total Isotropic Sensitivity (TIS), and correspond to the conductive-measured receiver sensitivity minus the total radiation efficiency of the antenna. This TIS can also be measured in a reverberation chamber, by averaging over mode stirred positions and polarizations, thereby corresponding to the measurements of radiation efficiency when the terminal is receiving.
There is therefore a need for an improved method and apparatus for measuring the performance of antennas as well as of complete mobile and wireless terminals, such as mobile phones, and specifically for measuring radiation efficiency, total radiated power, receiver efficiency and receiver sensitivity.